Probabilistic assessment of airblast variability and fatality risk estimation for explosive blasts in confined building spaces

Explosive blasts in confined building spaces, such as lobbies or foyers, can amplify blast loads. This article uses the computational fluid dynamics model ProsAir to estimate blast loads in a typical ground floor lobby of a commercial or government building. Monte-Carlo simulation is used to probabilistically model the effect that variability and uncertainty of charge mass and location, net equivalent quantity factor, temperature, atmospheric pressure and model errors have on airblast variability. The analysis then calculates the probability of casualties due to the effects of pressure and impulse, where human vulnerability due to the effects of pressure and impulse is a function of lung rupture, whole-body displacement or skull fracture (or the combination of the three). The terrorist threats considered are improvised explosive devices ranging in mass from 5 kg (backpack bomb) to 23 kg (suitcase bomb) detonated in various locations inside the building. As expected, blast pressure and fatality risks are dependent on the type of facade glazing (e.g. vulnerable glazing allows venting of the blast), improvised explosive device size and location. It was found that the mean fatality risk for a 23 kg terrorist improvised explosive device is 8.6%, but there is a 5% chance that fatality risks can exceed 20%. It was also found that a probabilistic analysis yielded lower mean fatality risks than a deterministic analysis. The effect of venting was also significant. Mean fatality risks increased by up to 10-fold if there was no venting (i.e. a bunker-like structure without windows), but reduced by about 30% for a fully vented structure (i.e. no windows). This probabilistic analysis allows decision-makers to be more aware of terrorism risks to building occupants, and how improved building design and security measures may ameliorate these risks.